The invention relates to the process of spinning fibers of silk and silk-like proteins from an admixture of solubilized silk or silk-like protein in an ionic liquid solvent.
Artificial spinning methods for silk fibroin have typically fallen into the categories of wet spinning and electrospinning. Wet spinning has the advantage of producing reelable fiber that can be drawn and tested for mechanical properties. Electrospinning can produce extremely fine fibers in the form of a non-woven mat, however mechanical testing of individual fibers is not feasible. These electrospun mats can be used as scaffolds for biomedical applications.
In previously reported results, spin dopes have been formed with silk fibroin dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (U.S. Pat. No. 5,252,285), hexafluoroacetone (HFA), formic acid, trifluoroacetic acid (TFA), and water mixed with polyethylene oxide (PEO). All of these spin dopes require a time-consuming, multistep process, where the silk fibroin is first washed of the glue-like sericin covering the fiber, dissolved in a salt based solvent, dialysed, and dried to eliminate the β-sheet crystal structure before the fibroin is soluble in the spinning solvent. It has been shown previously that silk fibroin is directly soluble in organic-based ionic liquids, both with or without the sericin present, as in U.S. patent application Ser. No. 11/326,678, incorporated herein by reference. These ionic liquid silk solutions were used to cast films of silk. As a spin dope, ionic liquids exhibit several advantages over the aforementioned spin dopes, such as negligible volatility and ease of solvent recovery.
The approach to wet spinning of silk fibroin from the silkworm Bombyx mori has varied in terms of equipment, i.e. a syringe pump with a hypodermic needle, a cylinder pressurized with N2 gas to force the spin dopes through a spinneret, and a syringe pump with a tapered spinneret etched in silicon to better control the fiber crystallite orientation. These spinning techniques typically utilize a methanol coagulation bath to remove the spinning solvent and induce crystallization in the fibers. Subsequent post-spin fiber drawing is typically conducted after soaking the fibers overnight in a methanol bath while the fibers were wet with methanol or water. Some fibers have been steam annealed. The drawing and annealing improve both the maximum modulus and the strain to break of the fibers.